AT1859U1 - SHORT PULSE LASER DEVICE - Google Patents

Abstract

In the described short pulse laser device (1) with passive mode coupling, with a laser resonator (9), to which a pump beam (3) is supplied, with a laser crystal (4), in particular a titanium sapphire (Ti: S) laser crystal, With an optical Kerr element, which is preferably formed by the laser crystal (4) itself, and with a diaphragm (5) that cuts into the resonator laser beam (8), the opening (20) of the diaphragm (5) has an adjustment to the beam cross section arcuate or polygonal edge (15).

Description

AT 001 859 Ul

The invention relates to a short-pulse laser device with passive mode coupling, with a laser resonator, to which a pump beam is fed, with a laser crystal, in particular a titanium sapphire (Ti: S) laser crystal, with an optical Kerr element, which is preferably by the laser crystal itself is formed, and with an aperture that cuts into the resonator laser beam.

Such laser devices are used on the one hand for scientific purposes, on the other hand they can be used in material processing, especially when fine structures have to be produced.

In the mode-locked state, instead of emitting a continuous laser light (cw operation - continuous wave), a laser delivers laser pulses by storing energy and then emitting it in pulses. The period of these pulses generally corresponds to the round trip time of the pulses in the laser resonator, for example, with a length of the linear resonator of 2 m, pulses are generated with a frequency of approximately 75 MHz; the laser light pulse passes through the laser resonator in both directions, which corresponds to a length of 4 m in the present example. For mode locking, a loss is periodically introduced (at the resonator rotation frequency) so that the laser starts to pulse. This leads to a peak power of the pulses, which is much larger (e.g. 100 kW to 200 kW) than the output power of the laser in cw mode (e.g. 150 mW to 300 mW).

A basic distinction can be made between two types of mode coupling:

With active mode coupling, a periodic loss is introduced with an active element, a modulator, which is supplied with energy from the outside via a driver.

This forces the laser to operate in the time intervals where there is little loss, whereas the laser can store energy in the time intervals where there are high losses.

Passive mode locking takes advantage of the effect of optical non-linearity in the resonator, i.e. an optically non-linear element is arranged in the path of the laser beam, which changes its optical properties, such as transmission or reflectivity, in proportion to the intensity of the laser beam. For example, the laser crystal can be used as a non-linear element or Kerr element themselves are used, which in combination with a linear loss element forms a so-called saturable absorber, in which the total loss becomes smaller and smaller the higher the intensity of the incident laser light. A fluctuation in the laser power results in a pulse that sees a much lower loss than the laser in cw mode (see also US Pat. No. 5,079,772 A). The laser body (solid-state laser) consists of a non-linear material, the optical thickness of which varies with the field strength distribution of the laser radiation. For example, the non-linear refractive index is a function of the square of the field strength, i.e. the laser beam, whose spatial field strength distribution can be assumed according to a Gaussian curve, " sees " in the case of a laser crystal having plan-parallel surfaces, effectively an element with an optical thickness that changes over the cross section. In this way, a focusing lens is created from a plane-parallel non-linearity.

This optical Kerr effect can be used in two ways for mode coupling (so-called " Kerr-lens mode-locking "): in the case of the so-called " soft aperture " (see Spence et al., Optics Letters, January 1, 1991, Vol.16, pp. 42-44), the pump beam (in the case of Ti: S lasers, the energy is supplied by means of a green laser, such as, for example, argon laser) focused into the laser crystal so that the resonator beam generated by the Ti: S laser (approx. 800 nm, infrared) can then absorb most of the pump energy, ie the highest gain is experienced when it has the smallest diameter. The higher the intensity or the field strength of the pulse, the more the laser pulse is focused, and the more it is amplified each time the laser crystal passes through, which in turn increases its intensity. This positive feedback leads to stable mode coupling.

In the case of the so-called hard diaphragm (eg US Pat. No. 5,079,772 A and US Pat. No. 5,097,471 A), the effect is exploited that a diaphragm cuts the resonator beam at a point where it has a larger diameter at the time when the Intensity (field strength) is lower, and has a 3 AT 001 859 ul smaller diameter at the time when the intensity is greater and the resonator beam is thus focused in the laser crystal. On such a laser device, namely with " hard " Aperture, the invention relates.

It is now the object of the invention to provide a laser device of the type mentioned at the beginning, in which a further increased output power and an additional shortening of the laser pulses, on pulse durations of the order of 10 fs and below, is made possible with simple means.

The laser device according to the invention of the type mentioned at the outset is characterized in that the opening of the diaphragm has an at least partially arcuate or polygonal edge.

The invention is based on the knowledge that with an appropriate adaptation of the diaphragm to the beam cross-section, an effective mode coupling is achieved even with a smaller linear loss in beam cross-sectional area, compared to conventional diaphragms which had straight-line slots as an aperture. It has been shown that a performance increase of more than 10% can be achieved.

Furthermore, by adapting the aperture to the beam cross section, a broadening of the mode-locked spectrum is made possible, such broadening in the spectral range being able to be used to generate even shorter laser pulses in the time range. Such a broadened spectrum of the output laser beam is also advantageous when applications of broad spectra are desired, e.g. for broadband telecommunications or in spectroscopy.

Depending on the cross-sectional shape of the laser beam, which can be influenced, for example, with the aid of the spherical mirrors or lenses provided in the resonator, it is advantageous if the aperture of the diaphragm has opposite elliptical arc edges or circular arc edges. This ensures advantageous aperture adjustments for the constriction of the respective resonator beam. For an optimal result with regard to increased performance and broadening of the spectrum, it is also advantageous if the diaphragm has two diaphragm parts that can be adjusted relative to one another - each having an arcuate or polygonal incision 4 AT 001 859 Ul as the opening edge.

In contrast to a diaphragm with: a simple straight-line slot, where an aperture adjustment in the slot direction can be superfluous, in the present laser device a fine adjustment of the aperture to the resonator beam in the sense of both a " horizontal " as well as a " vertical " Centering expediently (whereby "horizontal" and "vertical" here only in relation to, for example, a horizontal direction of polarization of the laser light, but should not be understood as restrictive information). In general, it is therefore advantageous in the present laser device for fine adjustment if the diaphragm can be adjusted in two mutually perpendicular directions.

Furthermore, in tests in connection with conventional laser devices, in particular with beam diameters in cw mode of approx. 2 mm to 2.5 mm, it has proven to be advantageous if the aperture in the direction of the polarization of the laser light has a, preferably adjustable, maximum width from 1 mm to 3 mm. Finally, for a particularly efficient constriction, it is also advantageous if the diaphragm is arranged at a distance of 1 cm or a few cm in front of the one end mirror of the laser resonator 9.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the drawing, to which, however, it is not intended to be limited. In detail: FIG. 1 shows a schematic of a short-pulse laser device in plan view; 2 shows an equivalent image of this laser device according to Fig.l; 3 shows a view of the aperture provided in this laser device; 4 shows a schematic view of the adjustment options for this diaphragm, the diaphragm being set to the greatest width; 5 schematically shows another extreme aperture setting, namely with a minimal aperture; and FIG. 6 shows a view of a modified diaphragm with a polygonal opening.

A short-pulse laser device 1 is schematically illustrated in FIG. 1, in which the aforementioned “Kerr-lens mode-locking11 principle” is used for short-pulse generation and laser mirrors implemented for thin-film technology for dispersion control. _ 5 AT 001 859 Ul

According to Fig.l, the laser device 1 has a laser head 2 indicated by a dashed line, to which a pump beam 3, e.g. an argon laser beam is supplied. The pump laser (e.g. argon laser) itself is omitted for the sake of simplicity and belongs to the state of the art.

After passing through a lens LI and a semitransparent laser mirror M2, the laser beam passes through a laser crystal 4, in the present example a titanium: sapphire solid-state laser crystal, after which it strikes a laser mirror Ml and from there to a laser mirror M3 outside the laser head 2, traversing one below aperture 5 to be explained in more detail, is reflected. This laser mirror M3 reflects the laser beam back to the mirror M1, from where the laser beam is reflected back to the laser mirror M2, passing through the laser crystal 4 a second time. From there, the laser beam is then reflected via further laser mirrors M4, M5 and a semi-transparent, wedge-shaped coupling mirror 6, whereby the laser resonator is formed. The laser beam is also coupled out via the coupling-out mirror 6, with a compensation plate 7 and mirrors M6, M7 using thin-film technology for dispersion compensation. The wedge shape of the coupling bar 6 ensures that no undesired reflections occur in the direction of the laser resonator from its rear side.

The resonator beam obtained in the described manner in the laser head 2 is denoted by 8 and the laser resonator is denoted by 9; the - " folded " - Laser resonator 9 a short resonator arm dl (between mirrors Ml and M3) and a long resonator arm d2 (between mirrors M2 and 6).

The laser crystal 4 is a plane-parallel body (a parallelepiped), which is optically non-linear and forms a Kerr element, which has a greater effective optical thickness for higher field strengths of the laser beam 8, but has a smaller effective thickness where the field strength or The intensity of the laser beam 8 is lower. This core effect known per se is used for self-focusing of the laser beam 8, i.e. the laser crystal 4 forms a focussing lens for the laser beam (resonator beam) 8 with a focal length that is dependent on the intensity of 6 AT 001 859 Ul.

The mirrors Ml to M7 are made using thin-film technology, i.e. they are each made up of many layers that perform their function in the reflection of the ultrashort laser pulse, which has a wide bandwidth. The various wavelength components of the laser beam penetrate the layers of the respective mirror to different depths before they are reflected. As a result, the different wavelength components are delayed at the respective mirror for different lengths; the short-wave components are reflected further outside, while the long-wave components are reflected more deeply in the mirror. As a result, the long-wave components are delayed compared to the short-wave components. In this way, dispersion compensation is obtained insofar as particularly short pulses (preferably in the range of 10 femtoseconds and below) have a broad frequency spectrum in the time domain; this leads to the fact that the different frequency components of the laser beam in the laser crystal 4 see a different refractive index, i.e. the optical thickness of the laser crystal 4 is different in size for the different frequency components, and the different frequency components are therefore delayed differently when passing through the laser crystal 4. This effect is countered by the dispersion compensation mentioned on the thin-film laser mirrors Ml to M7, cf. also Stingl et al. " Generation of 11-fs pulses from a Tiisapphire laser without the use of prisms ", Optics Letters, February 1, 1994,

Vol. 19, No. 3, pp. 204 - 206.

In view of the small delays per reflection from a mirror Ml to M7, it is expedient or necessary to use a comparatively thin laser crystal 4 which, on the other hand, should have a high doping in order to achieve the desired effect, the pump beam 3 and the resonator beam 8 are to be focused as strongly as possible.

Of course, other forms of dispersion compensation are also possible, such as using a pair of prisms or a Gires-Tournois interferometer; these elements also introduce a transit time difference between individual frequency components, but are based on different physical principles than the thin-film laser mirrors; these principles 7 AT 001 859 Ul are also known per se and therefore do not need to be explained further here.

The aperture 5 is used for mode coupling, which cuts the resonator beam 8 at a point where it has a larger diameter when the intensity or field strength of the fluctuating laser beam is lower, but when the intensity of the laser beam 8 is due to the fluctuation is larger, ie if the resonator beam 8 in the laser crystal 4 experiences self-focusing, has a smaller diameter. This mode coupling mechanism can be illustrated using the schematic equivalent image according to FIG. 2 for the folded resonator 9 illustrated in FIG.

As already indicated above, the mode coupling mechanism is therefore fundamentally based on the fact that the cross section of the laser beam 8 (the mode) in the laser resonator 9 changes. The mode is large in cross section in continuous wave mode (cw mode - continuously emitting laser), and this larger cross section is illustrated in FIGS. 2 and 3 with a broken line 10. In pulsed operation, the laser beam 8 has a significantly smaller diameter, which is illustrated in FIGS. 2 and 3 with a solid line 12.

In the replacement image of FIG. 2 for the folded resonator 9 according to FIG. 1, two spherical lenses 13, 14 also replace the spherical focusing mirrors Ml, M2 from FIG. 1, and the resonator 9 is a flat end mirror, for example mirrors 6 and M3 according to Fig.l, completed, one of these end mirrors, for example the mirror 6, is designed as a decoupling mirror. Furthermore, FIG. 2 also shows the laser crystal 4 used as a Kerr element for mode coupling according to the so-called “Kerr-lens mode-locking” principle.

In front of the end mirror M3 on the right in FIG. 2 there is the diaphragm 5, which supports the mode coupling mechanism on the principle of the hard diaphragm. This happens because - as can be clearly seen from Fig. 2 in particular also from Fig. 3 - the aperture 5 introduces a greater loss in continuous wave mode than in mode-locked, i.e. pulsed operation. by cutting the larger cross section 11 of the laser beam 8 relatively strongly in continuous wave operation 8 AT 001 859 Ul and in mode-locked operation allowing the smaller cross section 12 of the laser beam 8 to pass through more or less unhindered. A stable mode-locked state is thereby achieved.

The degree of mode locking, i.e. the proportion of the cw background in relation to the mode-locked laser line can be adjusted by a correspondingly stronger or weaker constriction of the laser beam 8 with the aid of the diaphragm 5. Normally, the aperture 5 should constrict the laser beam 8 just enough that there is no longer any cw component in the laser output signal.

The effect of constricting the resonator beam 8 is different in the horizontal direction and the vertical direction, which is related to the direction of polarization of the laser light. In the case of a horizontal polarization of the light of the laser resonator 9, that is if the field vector runs horizontally, the best effect of the diaphragm 5 is given when the laser beam 8 is constricted in the horizontal direction, as is shown in FIGS. 1 to 3 . This orientation relationship is referred to below by way of example, but without this however always aligning the diaphragm (here an upright diaphragm, see FIG. 3, which shows a view in the diaphragm area, whereas FIGS. 1 and 2 illustrate schematic plan views of the laser device) must be given - of course, the orientations of the diaphragm 5 must be changed accordingly in a different laser light polarization direction, in order to again achieve the optimal diaphragm effect.

In previous laser devices, a simple slit diaphragm was used, which had two straight, parallel slit edges. However, this linear incision of the diaphragm edges, which cut away lateral segments in the cross section of the resonator beam (in cw mode), gave rise to various disadvantageous effects, such as, in particular, the use of less than optimal power.

In the present laser device, the resonator beam 8, which is round to elliptical in cross section, is constricted by an aperture 5 adapted to it. The plane-parallel cutting edges of the previous panels are replaced by arc-shaped, in particular circular-arc cutting edges or opening edges 15, cf. Fig. 3. The diaphragm 5 itself consists, for example, of 9 AT 001 859 Ul from two diaphragm parts 5a, 5b which can be adjusted relative to one another in the horizontal direction (or generally parallel to the polarization direction of the laser light), at least one of the diaphragm parts 5a, 5b being horizontally adjustable, preferably as is apparent from the double arrows 16, 17 in FIG. 4 shows that both diaphragm parts 5a, 5b can be adjusted horizontally and independently of one another. 4 shows the setting of the aperture 5 with the maximum width or aperture.

For the horizontal centering of the diaphragm 5 there is also a horizontal displacement of the entire diaphragm 5, according to arrow 18, and for the vertical centering of the diaphragm 5 with respect to the laser beam 8, a vertical displacement of the diaphragm 5 as a whole, according to arrow 19 in FIG. 4, provided. This vertical displacement of the diaphragm 5 (or generally at right angles to the direction of polarization of the laser light) is, in contrast to the conventional diaphragms with a vertical slit, expedient in the present laser device because the edges 15 of the opening 20 of the diaphragm 5 are curved, for example elliptical or especially circular arc-shaped, so that fine adjustment to the laser beam 8 is expedient in this vertical direction in order to be able to take full advantage of the effects which can be achieved by the arcuate edges 15 when constricting the laser beam 8. For the adjustment of the diaphragm 5 or diaphragm parts 5a, 5b according to arrows 16 to 19, conventional mechanisms, such as in particular spindle drives or micrometer screw gears, with corresponding guides and bearings can be provided. Since this is a conventional technology per se, a detailed description of these adjustment mechanisms can be dispensed with here.

The shape of the cross section of the resonator beam 8 can be influenced with the aid of the mirrors Ml, M2 according to FIG. 1 by their angular position, with, for example, as is preferred in the present laser device, an elliptical cross section in cw mode in the area of the aperture 5 ( s. the cross-sectional line 11 in FIG. 3) can be set; the main axis of the ellipse runs horizontally or generally parallel to the polarization vector of the laser light. If now, as shown in Fig. 3, the diaphragm parts 5a, 10 AT 001 859 Ul 5b are mutually adjusted such that the opening edges define 15 arcs, i.e. lie on one and the same circle, a circular cross section of the laser can be achieved in the constriction, as illustrated by the full line 12 in FIG. In general, the arc shape or the radius of curvature r (FIG. 3) of the edges 15 should be adapted to the beam diameter in order to achieve the best possible results with regard to increasing the output power and broadening the mode-locked spectrum. In experiments, it was found that for a laser resonator with a length of approx. 2 m and focusing mirrors Ml, M2 with a radius of -50 mm, the beam diameter at the location of the aperture 5 near the end mirror M3 of the shorter resonator arm (dl in Fig.l) in continuous wave mode is approximately 2 mm to 2.5 mm, and that the beam diameter in mode-locked operation is reduced to approximately 1 mm to 1.5 mm. With such a laser dimensioning, optimal results are achieved if the edges 15 are defined in the shape of a circular arc, with a radius r of approximately 1.5 mm.

Furthermore, it has also been found that the diaphragm 5 is arranged relatively close to the end mirror M3 of the short resonator arm d1, namely at a distance of approximately 1 cm or a few cm, for example 1 cm to 8 cm, depending on the length of the resonator arm . As a guideline, it was found here that the distance of the diaphragm 5 from the end mirror M3 can be approximately 2% to 5% of the length of the short resonator arm d1 (which is, for example, approximately 80 cm).

Experiments have shown that with such an aperture 5, the spectrum can be broadened by approximately 15%. Such broadening in the frequency domain means that correspondingly shorter pulses can be achieved in the time domain. Furthermore, a wide output spectrum of the laser device can be better used for all applications of wide spectra.

Furthermore, it was found that the output power of the mode-locked laser beam could be increased by adapting the aperture 5. The efficient mode coupling is evidently achieved with a smaller loss of beam cross-sectional area compared to the conventional straight slit diaphragms. The power increases achieved in tests were over 10% at 11 AT 001 859 Ul, and for example average powers of over 500 mW and more (instead of 450 mW previously) could be achieved. The peak power, which is greater the shorter the pulses are, could be increased to over 400 kW with pulse durations of 10 fs, at 75 MHz pulse repetition frequency, with increases of up to 1 MW seeming possible.

In Figure 5, the narrowest aperture setting is shown compared to Figure 4; 4 and 5 show two extreme diaphragm settings for the purpose of illustrating diaphragm setting options, by moving the diaphragm parts 5a, 5b relative to one another, FIG. 4, for example, a circular diaphragm opening (with a diameter of 3 mm in the numerical example above) and Fig. 5 shows an aperture narrowed to 1/3 (with a maximum width of 1 mm in the numerical example mentioned).

As FIG. 6 shows, if a simple manufacture is preferred over an ideal adaptation, the screen 5 can also be designed with a polygonal opening 20, the screen parts 5a ', 5b' having corresponding polygonal incisions or opening edges; Here, too, an adaptation to the beam cross section can be achieved, although this is not quite as effective as in the case of a circular or elliptical aperture. 12th

Claims (7)

AT 001 859 Ul Claims: 1. Short-pulse laser device (1) with passive mode coupling, with a laser resonator (9), to which a pump beam (3) is supplied, with a laser crystal (4), in particular a titanium sapphire (Ti: S-) laser crystal, with an optical Kerr element, which is preferably formed by the laser crystal (4) itself, and with an aperture (5) which cuts into the resonator laser beam (8), characterized in that the opening (20) of the aperture ( 5) has an at least partially arcuate or polygonal edge (15). 2. Laser device according to claim 1, characterized in that the opening (20) of the diaphragm (5) has opposite elliptical arc edges (15). 3. Laser device according to claim 1 or 2, characterized in that the opening (20) of the diaphragm (5) has opposite circular arc edges (15). 4. Laser device according to one of claims 1 to 3, characterized in that the diaphragm (5) has two relatively adjustable diaphragm parts (5a, 5b), each having an arcuate or polygonal incision as the opening edge (15). 5. Laser device according to one of claims 1 to 4, characterized in that the diaphragm (5) in two mutually perpendicular directions (18, 19) is adjustable. 6. Laser device according to one of claims 1 to 5, characterized in that the diaphragm (5) in the direction of polarization of the laser light has a, preferably adjustable, maximum width of 1 mm to 3 mm. 7. Laser device according to one of claims 1 to 6, characterized in that the diaphragm (5) is arranged at a distance of 1 cm or a few cm in front of the one end mirror (M3) of the laser resonator (9). 13